tdo

commit c53dbc278c89a74361f379a21e76fe028686a475
parent eef434e314ef6b71370c426e7e443f19d29b2d2d
Author: david cochran <about.trout@gmail.com>
Date:   Mon, 31 Jan 2022 02:47:35 +0000

add layer2 solution

Diffstat:
M
bitwise.go
 | 
21
+++++++++++++++++++++
M
bitwise_test.go
 | 
40
++++++++++++++++++++++++++++++++++++++++
A
cmd/layer2/decode.go
 | 
25
+++++++++++++++++++++++++

3 files changed, 86 insertions(+), 0 deletions(-)
diff --git a/bitwise.go b/bitwise.go
@@ -12,3 +12,24 @@ func FlipSecondBits(b byte) byte {
 func CycleRight(b byte) byte {
 	return (b << 7) | (b >> 1)
 }
+
+func ParityCheck(b byte) bool {
+	p := bits.OnesCount8(b&0b11111110) % 2
+	return p == int(b%2)
+}
+
+func PackDataBytes(bs []byte) []byte {
+	if len(bs)%8 != 0 {
+		log.Fatalf("Failed to PackDataBytes: input slice of length %d, not a multiple of 8", len(bs))
+	}
+	var out []byte
+	for i := 0; i < len(bs)/8; i++ {
+		for j := 0; j < 7; j++ {
+			k := i*8 + j // index in bs
+			left := (bs[k] >> 1) << (j + 1)
+			right := bs[k+1] >> (7 - j)
+			out = append(out, left|right)
+		}
+	}
+	return out
+}
diff --git a/bitwise_test.go b/bitwise_test.go
@@ -33,3 +33,43 @@ func TestFlipSecondBits(t *testing.T) {
 		}
 	}
 }
+
+func TestParityCheck(t *testing.T) {
+	tests := []struct {
+		in   byte
+		want bool
+	}{
+		{0b10100011, true},
+		{0b10100010, false},
+
+		{0b11111111, true},
+		{0b11111110, false},
+
+		{0b00000000, true},
+		{0b00000001, false},
+	}
+
+	for _, test := range tests {
+		if got := ParityCheck(test.in); got != test.want {
+			t.Errorf("ParityCheck(%08b) = %t; want %t", test.in, got, test.want)
+		}
+	}
+}
+
+func TestPackDataBytes(t *testing.T) {
+	in := []byte{
+		0b11111110, 0b11111110, 0b11111110, 0b11111110,
+		0b11111110, 0b11111110, 0b11111110, 0b11111110,
+		0b11111110, 0b11111110, 0b11111110, 0b11111110,
+		0b11111110, 0b11111110, 0b11111110, 0b11111110,
+	}
+	want := []byte{
+		0b11111111, 0b11111111, 0b11111111, 0b11111111,
+		0b11111111, 0b11111111, 0b11111111, 0b11111111,
+		0b11111111, 0b11111111, 0b11111111, 0b11111111,
+		0b11111111, 0b11111111,
+	}
+	if got := PackDataBytes(in); !bytes.Equal(got, want) {
+		t.Errorf("PackDataBytes(%#v) = %#v (len %d); want %#v (len %d)", in, got, len(got), want, len(want))
+	}
+}
diff --git a/cmd/layer2/decode.go b/cmd/layer2/decode.go
@@ -0,0 +1,25 @@
+package main
+
+import (
+	"log"
+	"os"
+
+	"github.com/abtrout/tdo"
+)
+
+func main() {
+	bs, err := tdo.DecodePipedInput()
+	if err != nil {
+		log.Fatalf("Failed to read layer input: %v", err)
+	}
+	var filtered []byte
+	for _, b := range bs {
+		if tdo.ParityCheck(b) {
+			filtered = append(filtered, b)
+		}
+	}
+	out := tdo.PackDataBytes(filtered)
+	if _, err := os.Stdout.Write(out); err != nil {
+		log.Fatalf("Failed to write decoded output: %v", err)
+	}
+}